RF welding is a process that relies on internal heat generation by dielectric hysteresis losses of thermoplastics. Under a high-frequency electric field, a polar polymer undergoes a dipole polarization process forming strong dipoles. These dipoles tend to orient in the direction of the field being applied and try to continually align with the rapidly reversing, high-frequency electric field. Because of the bulky polymer chains and chain entanglement, the attempted alignment causes internal molecular friction and results in heat generation. The heat melts the surfaces of the two parts being joined and increases the polymer mobility of these surfaces. Ultimately, the polymer chains diffuse through the interface of these parts and become entangled to form a strong weld.
RF welding has traditionally been used to weld two flat-die extruded thermoplastic films together. This was accomplished by directing a powerful high-frequency wave through a shaped electrode that was made from bent extruded bars or machined brass. This generally produced flat, two-dimensional (2-D) shapes, such as an IV bag. Some additional shapes, such as welding half-circles around tubes, is also common, but not easy. The only way to affect shapes in three dimensions was to weld them in a flat, 2-D shape, and then pull them into 3-D by filling them or putting them through additional operations. This limited the application of RF welding to simpler constructions.
Typical RF welding equipment comprises three major components: 1) an RF generator; 2) a press; and 3) a set of dies, or electrodes. The generator commonly provides power ranging from 1 to 25 kW, depending on the welding area, part thickness, and the dielectric properties of the material being welded. Solid-state rectifiers in the power supply convert incoming alternating current into high-voltage direct current, followed by an oscillator converting high-voltage direct current into high voltage alternating current. The frequencies used for RF welding range from 13 to 100 MHz, but mot typically 27.12 MHz.
The press, usually a pneumatic press, consists of one fixed, lower platen and one movable, upper platen. During the welding process, the press moves the upper platen down and applies force onto respective sections of two parts to be joined which lie between the platens. Pressure is maintained during a hold cycle. There is usually a pressure regulator which determines the maximum force applied to the press.
The set of dies, or electrodes, include an upper fixture and a lower fixture. In the simplest tools, the upper fixture is a raised projection of metal that matches the geometry of the parts being welded and the lower fixture is a flat metal plate. This type of fixture is easy to fabricate and maintain, but this has two main disadvantages: 1) fringing of the electric field that produces melting beyond the upper electrode area; 2) the application whose parts are being welded must be able to accommodate a flat lower fixture.
In applications where cosmetics are important and/or the application shapes do not allow for a flat lower fixture, the lower fixture may also have a raised electrode protrusion, thereby allowing parts with tall dimensions to be welded. This type of fixture defines the electric field better and produces less fringing, and thus results in a more defined weld area, as compared to a flat lower fixture. However, alignment of the upper and lower fixtures is critical.
In automated systems, the lower fixture can translate the part in and out of the machine with a rotary station or with a slide shuttle table. The upper fixture, with a particular contour for the application, is attached to the moveable, upper platen of the press. The fixture supports the electrode that is used to apply the electric field and localized clamp force to assure proper welding. The electrodes are typically fabricated from 2 to 4 mm thick copper, bronze, or brass sheet metal.
In fabricating a fixture, it is possible to machine the raised electrode from a solid plate of metal, however it is more common to bend the electrodes from a standard electrode profile. Commercially available profiles are usually made of brass or copper to maximize electrical and thermal conductivity. In addition, these materials are easy to machine. The profiles are pre-drilled to facilitate attachment to the lower plate and offer various surface finishes, such as flat, knurled, cut and seal profile, and stitched. It is worthwhile to note that, while many of the profiles have complex geometry, they do not have any sharp points that can concentrate the electric field and promote arcing or localized heating.
It is also common for the lower fixture (flat plate portion) to be covered with a non-stick, high-dielectric film, typically made of release paper, KAPTON® films, and TEFLON® films to help release the parts from the fixture, increase equipment efficiency and power delivery.
In applications that have high duty cycles or high production rates, the fixture can be water-cooled to prevent heat build-up and to minimize cycle times. When welding crystalline materials, which have a relatively distinctive melting, or processing, temperature, the fixtures are sometimes slightly heated with an electrical heater to minimize cycle times.
Applied voltage, electrode separation, and the localized clamp pressure are three critical process parameters for RF welding which must be taken into account in order to make welds that are strong and uniform. The greater the voltage, the higher the intensity of the electric field, and a correspondingly faster welding process. However, high voltage has the potential to break down the material. The appropriate voltage should be determined based on the electrode separation, and on the dielectric constant and break down of the material being welded. Heat generation is inversely proportional to the square of the electrode separation. Therefore, if the joint is too thick, the welding will not be effective. The typical thicknesses for RF welding are from 0.50 mm to 1.90 mm. The clamp pressure not only affects the electrode separation, but also facilitates melting and welding. Over-clamping, however, causes flashing or melt break down. As a result, any change in these parameters causes significant changes in welding properties. Keeping the conditions the same at all the locations of the area being welded is the key to producing uniform welds. Because the process is very sensitive to these conditions, RF welding processes are generally suitable for flat thin films or sheets, such as medical disposable bags, blister packs, and book/cassette covers.
In most applications, RF welding is utilized for sealing thin and flat parts. In some cases, however, parts to be welded may not be flat, such as in an inflatable CPAP (continuous positive airway pressure) mask interface. In this case, if the part was designed using typical RF welding standards, a flat part could be produced, whose perimeter would be made from the lay-flat dimensions. Lay-flat dimensions can be modeled by taking the 3-D shape of the finished product, and pushing them into a flat plane. However, since the component only functions in 3-D, the resulting part would need to be folded or inflated from the flat form in which it was welded into the desired 3-D shape. Since the component is made from normally flat films, when they are pulled or folded into 3-D, the film will naturally wrinkle, and produce an uneven surface. In the case of the CPAP mask, this would result in non-functional parts.
There is a need for a technique which can enable 3-D electrodes for use in RF welding processes to be manufactured and that will still follow RF tooling design standards, enabling 3-D parts to be welded with the material compressed under a constant pressure at all points along the weld, thereby producing welds with excellent strength and good cosmetic results.